In an internal combustion engine mounted on a vehicle or the like, a cylinder head is attached to a cylinder block, and a cylinder head cover is attached to the cylinder head. In the cylinder head, intake ports and exhaust ports are formed which communicate with a combustion chamber, and intake and exhaust valves are mounted for opening and closing the intake and exhaust ports, respectively.
According to one conventional type of internal combustion engine, intake-side tappet boss portions and exhaust-side tappet boss portions are formed in the cylinder head, and the intake and exhaust-side tappet boss portions axially movably support intake tappets for the intake valves and exhaust tappets for the exhaust valves, respectively. Intake-side bolt boss portions and exhaust-side bolt boss portions are also formed in the cylinder head, into which intake-side head bolts and exhaust-side head bolts are inserted respectively. The intake and exhaust-side bolt boss portions are adjacently positioned respectively on both sides of the intake-side tappet boss portions and the exhaust-side tappet boss portions in an axial direction of a crankshaft of the engine.
FIGS. 4 and 5 illustrate two such conventional cylinder head structures.
In FIG. 4, the numeral 102 denotes a cylinder head of an internal combustion engine (not shown). In the cylinder head 102, intake-side tappet boss portions 104 and exhaust-side tappet boss portions 106 are formed, and intake-side bolt boss portions 108 and exhaust-side bolt boss portions 110 are also adjacently formed respectively on both sides of the intake-side tappet boss portions 104 and the exhaust-side tappet boss portions 106 in the direction of crank axis L1. Intake-side tappet holes 112 and exhaust-side tappet holes 114 are formed in the intake-side tappet boss portions 104 and the exhaust-side tappet boss portions 106, respectively. The intake-side tappet holes 112 and exhaust-side tappet holes 114 axially movably support intake tappets and exhaust tappets for intake valves and exhaust valves which open and close intake ports and exhaust ports, respectively, (not shown). Intake-side bolt insertion holes 116 and exhaust-side bolt insertion holes 118 are formed in the intake-side bolt boss portions 108 and the exhaust-side bolt boss portions 110, respectively, into which insertion holes 116 and 118 are inserted intake-side head bolts and exhaust-side head bolts, respectively, for mounting the cylinder head 102 to a cylinder block (not shown).
In the cylinder head 102, the centerline-to-centerline spacings BP (bolt pitch) between the intake-side bolt boss portions 108 and between the exhaust-side bolt boss portions 110 are small, and innermost edge distances B between the intake-side bolt boss portions 108 and between the exhaust-side bolt boss portions 110 are smaller than outermost edge distances A between the intake-side tappet boss portions 104 and between the exhaust-side tappet boss portions 106 (i.e. A&gt;B). D (in FIGS. 4 and 5) represents the diameter of the intake-side bolt boss portions 108 and the exhaust-side bolt boss portions 110, and thus B=BP-D.
In the cylinder head 102, therefore, if an axis L2 joining the centers of the intake-side tappet boss portions 104 and an axis L4 joining the centers of the intake-side bolt boss portions 108 are positioned coaxially with one another and in parallel with the crank axis L1, and likewise if an axis L3 joining the centers of the exhaust-side tappet boss portions 106 and an axis L5 joining the centers of the exhaust-side bolt boss portions 110 are positioned coaxially with one another and in parallel with the crank axis L1, the intake-side tappet holes 112 and the intake-side bolt insertion holes 116 interfere with each other, and likewise the exhaust-side tappet holes 114 and the exhaust-side bolt insertion holes 118 interfere with each other. To avoid such interference, in the cylinder head 102, the axes L2 and L3 are positioned inside the axes L4 and L5, respectively, in a direction transverse to the direction of the crank axis L1 so that the intake-side tappet boss portions 104 and the exhaust-side tappet boss portions 106 are positioned inside the intake-side bolt boss portions 108 and the exhaust-side bolt boss portions 110, respectively, in the transverse direction.
On the other hand, in the conventional cylinder head 102 shown in FIG. 5, the spacings BP between the intake-side bolt boss portions 108 and between the exhaust-side bolt boss portions 110 are small, so an axis L2 joining the centers of intake-side tappet holes 112 is positioned outside an axis L4 joining the centers of intake-side bolt insertion holes 116 in a direction transverse to the direction of crank axis L1, and likewise an axis L3 joining the centers of exhaust-side tappet holes 114 is positioned outside an axis L5 joining the centers of exhaust-side bolt insertion holes 118 in a direction transverse to the direction of crank axis L1. Consequently, intake-side tappet boss portions 104 are positioned outside intake-side bolt boss portions 108 in the transverse direction, and exhaust-side tappet boss portions 106 are positioned outside exhaust-side bolt boss portions 110 in the transverse direction, to avoid interference.
Certain types of internal combustion engines have what are called top entry ports (TEP); that is, intake ports which communicate with a combustion chamber and open in the upper surface of a cylinder head. This type of cylinder head for an internal combustion engine, as discussed above, includes intake-side tappet boss portions and exhaust-side tappet boss portions, and intake-side bolt boss portions and exhaust-side bolt boss portions adjacently located on both sides of the intake and exhaust-side tappet boss portions, respectively, in a crank axis direction.
Such cylinder head structures of internal combustion engines are disclosed in Japanese Patent Laid-Open Nos. 2-81919, 7-63118 and 7-305652.
According to the cylinder head structure disclosed in Japanese Patent Laid-Open No. 2-81919, intake valve ports and exhaust valve ports are formed in a head block of each cylinder along the axis of a crankshaft, and at least one of the intake ports and exhaust ports extending from those valve ports are drawn out upwardly.
According to the cylinder head structure disclosed in Japanese Patent Laid-Open No. 7-63118, both intake and exhaust ports are opened in one side wall portion of a cylinder head so that the intake ports are positioned above and in parallel with the exhaust ports, and stem portions of the exhaust valves are passed through both exhaust and intake ports.
According to the cylinder head structure disclosed in Japanese Patent Laid-Open No. 7-305652, a combustion chamber is formed on the bottom of a cylinder head, and intake and exhaust passages are formed in the cylinder head so as to open at one end into the combustion chamber, the opposite end of each of the intake passages opening at the upper surface of the cylinder head, and the opposite end of each of the exhaust passages opening at the bottom of the cylinder head.
In internal combustion engines provided with a cylinder head having so-called top entry ports (TEP) as intake ports formed in the upper surface of the cylinder head which communicate with a combustion chamber, as referred to above, if the head bolt pitch in the cylinder block is set small with a view to shortening the overall length of the internal combustion engine, it becomes difficult to ensure a space for the formation of an intake port in the upper surface of the cylinder head. On the other hand, if an attempt is made to ensure a space for the formation of an intake port in the upper surface of the cylinder head, such an attempt brings about a disadvantage from the standpoint of both space and weight.
For example, as shown in FIG. 4, in the cylinder head 102 wherein the intake and exhaust-side tappet boss portions 104 and 106 are respectively positioned inside the intake and exhaust-side bolt boss portions 108 and 110 in the transverse direction, an upper surface 120 of the cylinder head located between the intake and exhaust-side tappet boss portions 104 and 106 is narrow, and a spark plug hole 122 occupies this space, so that it is difficult to ensure a space for the formation of intake ports.
On the other hand, in the cylinder head 102 shown in FIG. 5 wherein the intake and exhaust-side tappet boss portions 104, 106 are respectively positioned outside the intake and exhaust-side bolt boss portions 108, 110 in the transverse direction, the upper surface 120 of the cylinder head located between the intake and exhaust-side tappet boss portions 104 and 106 is wide, so that it is possible to ensure the space for the formation of intake ports. In this case, however, the width of the cylinder head 102 itself becomes large, which is disadvantageous from the standpoint of both space and weight.
If priority is given to ensuring a space for the formation of intake ports despite the disadvantage in both space and weight, taking into account the point that the space of engine room is to be narrowed and the point that fuel economy of the internal combustion engine is to be attained, and if along this intention there are formed intake ports in the upper surface 120 of the cylinder head 102 shown in FIG. 4, the internal combustion engine can be rendered compact and at the same time it is possible to form intake ports, but the shape of the intake ports is restricted due to interference with the intake and exhaust-side tappet boss portions 104 and 106 and it becomes difficult to introduce air into the combustion chamber, which is disadvantageous to the performance of the internal combustion engine. Additionally, it becomes difficult to ensure a mounting space of an intake manifold, and a restriction is imposed on the layout of the intake manifold.